ATF Final Rule 2021R-05F, formally the “Definition of ‘Frame or Receiver’ and Identification of Firearms,” took effect on August 24, 2022, and remains fully enforceable after the Supreme Court upheld it in March 2025.1Bureau of Alcohol, Tobacco, Firearms and Explosives. Definition of Frame or Receiver and Identification of Firearms2Congressional Research Service. Supreme Court Upholds ATF Ghost Gun Regulation in Bondi v VanDerStok The rule modernizes what federal law counts as a firearm frame or receiver, brings unfinished parts and build kits under the Gun Control Act when they can readily be completed into working firearms, requires federal firearms licensees to serialize privately made firearms that come into their inventory, and restores permanent retention of dealer transaction records. If you build guns at home, buy or sell parts kits, or hold an FFL, this is the rule that governs what you do.
What Now Counts as a Frame or Receiver
The old definition in 27 CFR was written decades ago and didn’t map onto modern designs. Striker-fired pistols and AR-platform rifles don’t have a single component that houses every part the old regulation described, which caused years of classification disputes. The updated 27 CFR 478.12 defines a frame or receiver as the part of a weapon that provides housing or a structure for the primary components that make the weapon fire, such as the hammer, bolt, breechblock, or firing mechanism.3eCFR. 27 CFR 478.12 – Definition of Frame or Receiver
For split designs like AR-15 and M-16 variants, ATF designates the lower receiver as the regulated part because it houses the trigger mechanism and hammer. When a multi-piece frame or receiver breaks down into modular subparts, the subpart carrying the manufacturer’s serial number is presumed to be the regulated component. Prior ATF classifications for common firearms issued before April 26, 2022 remain in force.
Silencers get their own treatment. On a standard single-tube design, the frame or receiver is the outer tube or housing containing the baffles or expansion chamber. On a modular design, it is the principal housing that attaches directly to the weapon. Removable end caps are excluded.4Bureau of Alcohol, Tobacco, Firearms and Explosives. Definition of Frame or Receiver and Identification of Firearms – Final Rule Overview Silencer parts sold separately to anyone who is not a Special Occupational Taxpayer must be marked, registered, and transferred under the National Firearms Act.
The “Readily” Standard and Parts Kits
This is the change that reshaped the market for 80% receivers and build kits. If a part is designed to or can readily be completed, assembled, or converted to function as a frame or receiver, federal law treats it the same as a finished one.4Bureau of Alcohol, Tobacco, Firearms and Explosives. Definition of Frame or Receiver and Identification of Firearms – Final Rule Overview
There is no bright-line time threshold. ATF applies a multi-factor analysis and considers any jigs, templates, molds, tools, equipment, instructions, guides, or marketing materials sold alongside the part or made available by the seller. A partially machined receiver blank sold bundled with a jig and finishing instructions looks nothing like a raw block of aluminum sold with nothing else, and the rule treats them differently.
ATF’s own examples set the boundaries. A partially completed AR-15 receiver blank sold with a compatible jig is a receiver, because someone with common hand tools and online instructions can finish it without specialized skill. A raw billet with no indexing, machining, or forming on critical interior areas, sold without jigs or instructions, is not. The Supreme Court pointed to a Polymer80 “Buy Build Shoot” kit requiring roughly 20 minutes of assembly with common tools as an obvious example of a readily convertible product.5Supreme Court of the United States. Bondi v VanDerStok Opinion
A parts kit that can readily be assembled into a working firearm is itself a firearm under federal law, even if the frame inside the kit is technically unfinished at the point of sale. Splitting the frame and the finishing tools into separate shipments does not help if the items are marketed together. Commercial sellers of regulated kits must hold a Federal Firearms License and run purchasers through the National Instant Criminal Background Check System.1Bureau of Alcohol, Tobacco, Firearms and Explosives. Definition of Frame or Receiver and Identification of Firearms Selling without a license exposes the seller to prosecution under 18 U.S.C. § 922.
What You Can Still Do as a Personal Builder
The rule does not stop you from building a firearm for your own use. If you make a firearm for yourself and do not intend to sell or distribute it, you do not need a manufacturer’s license, and you are not required to add a serial number or register it.6Bureau of Alcohol, Tobacco, Firearms and Explosives. Privately Made Firearms
The line moves the moment you build with intent to sell. A “manufacturer” under the Gun Control Act is anyone engaged in the business of making firearms for sale or distribution, and even occasional sales pull you across that line. The same rule applies to gunsmiths: finishing work on firearms that will be offered for sale is manufacturing.7Bureau of Alcohol, Tobacco, Firearms and Explosives. NFA Handbook – Chapter 7 Manufacturing NFA Firearms
A few hard limits apply to personal builds regardless of intent. The finished firearm must be detectable under federal law, meaning it cannot be made entirely of materials that would pass through a metal detector unnoticed. Privately making silencers, destructive devices, or machine gun conversion devices without proper federal licensing is illegal and carries up to 10 years in prison. State and local laws may add restrictions, so check your jurisdiction before starting.
Serialization When a Privately Made Firearm Reaches an FFL
The serialization mandate is not aimed at the individual builder. It attaches when a privately made firearm enters the commercial stream. Whenever an FFL receives a privately made firearm for any reason, whether purchase, trade, consignment, or repair, the licensee must mark it with a unique serial number.
Format and Marking
The serial number must begin with the FFL’s abbreviated license number (the first three and last five digits), followed by a hyphen and a unique identifier that has not been used on any other firearm handled by that licensee. An example format is “12345678-001.”8eCFR. 27 CFR 478.92 – Firearms; Marking Requirements
Markings must be engraved, cast, or stamped to a minimum depth of .003 inches, with a print size no smaller than 1/16 inch, legible to the naked eye, and using only Roman letters, Arabic numerals, or hyphens. On polymer-framed firearms, the serial number may be embedded on a permanently affixed metal plate. The marking must also include the FFL’s name and the firearm’s caliber or gauge.
Timing, Exemptions, and Cost
The licensee must apply the serial number within seven days of receiving the firearm or before transferring it to another person, whichever comes first. If an FFL adjusts or repairs a privately made firearm and returns it to the owner on the same business day, the licensee does not need to take it into inventory or serialize it.9Bureau of Alcohol, Tobacco, Firearms and Explosives. Summary of Final Rule 2021R-05F
Several categories fall outside serialization entirely. The requirement does not apply to firearms manufactured before October 22, 1968, unless remanufactured since; to firearms already identified and registered in the National Firearms Registration and Transfer Record under the NFA; or to unlicensed individuals marking their own personal builds.10Bureau of Alcohol, Tobacco, Firearms and Explosives. Training Aid for the Definition of Frame or Receiver and Identification of Firearms
FFL serialization services for privately made firearms typically cost between $30 and $200, with polymer frames requiring embedded metal plates running toward the higher end.
Permanent Record Retention for FFLs
The rule ends the 20-year destruction option that had been in place since 1985. Licensees must keep all acquisition and disposition records, along with ATF Forms 4473, until they discontinue the business or licensed activity. No records may be destroyed while the license is active. When a business closes, the records go to the ATF National Tracing Center.11ATF eRegulations. 27 CFR 478.129 – Record Retention
For privately made firearms, the newly assigned serial number goes into the acquisition and disposition book at the same time the physical marking is applied, linked to the person from whom the firearm was acquired.
Willfully making false entries in these records is a federal crime punishable by up to one year in prison.12Office of the Law Revision Counsel. 18 USC 924 – Penalties ATF may also revoke a dealer’s license for willful violations of the Gun Control Act or its regulations, and for most licensees that is the more consequential outcome.13Office of the Law Revision Counsel. 18 USC 923 – Licensing
Electronic Storage of Forms 4473
Electronic retention is permitted, but the system must meet the requirements in ATF Ruling 2022-01. A licensee planning to switch must notify the local ATF Industry Operations Area Office in writing at least 60 days in advance.14Bureau of Alcohol, Tobacco, Firearms and Explosives. ATF Ruling 2022-01 – Electronic Storage of Forms 4473
Forms must be stored in an unalterable format. The original cannot be deleted, amended, or replaced; corrections go on an attached copy. The system must give ATF read-only access for inspections, trace requests, and investigations, and must provide at least one computer terminal for every 500 forms executed in the prior 12 months, up to a maximum of five. Licensees using cloud or remote server storage must also maintain a local backup at the licensed premises, updated the same day any record changes.
If the business closes, electronically stored forms must be delivered to the National Tracing Center in an image format such as TIFF, JPEG, or PDF, with optical character recognition disabled and images flattened so they are not electronically searchable.
Where the Rule Stands After Bondi v. VanDerStok
The rule survived its main legal challenge. On March 26, 2025, the Supreme Court ruled 7–2 in Bondi v. VanDerStok (Docket No. 23-852) that 2021R-05F is consistent with the Gun Control Act.15Legal Information Institute (LII). Garland v VanDerStok5Supreme Court of the United States. Bondi v VanDerStok Opinion Justices Thomas and Alito dissented. Every major provision of the rule is enforceable nationwide, and manufacturers, dealers, and builders should treat it as settled law unless Congress passes superseding legislation.